It has been 150 years since the epidemiologist John Snow traced the source of an epidemic of cholera to the Broad Street pump in London. Yet, despite Snow's acuity, certain parts of the world are still blighted by cholera epidemics that occur with an as-yet-unexplained seasonal regularity.

Now, the combined efforts of scientists from Bangladesh, India and the United States have revealed that epidemics of cholera are inversely correlated with the prevalence of cholera phages in contaminated water, and these findings are reported in Proceedings of the National Academy of Sciences USA.

John Mekalanos and co-workers analysed samples from water sources in Dhaka, Bangladesh — a city where cholera epidemics occur during certain months every year. The researchers found a statistically significant inverse correlation between the level of virulent cholera phages and that of phage-susceptible epidemic Vibrio cholerae strains O1 and O139 in sampled water. Importantly, when the time of onset of cholera epidemics was compared with the concentration of cholera phages in these samples, a striking pattern was observed — cholera epidemics that were caused by either the O1 or O139 serogroup strains usually commenced when low levels of O1 or O139-specific cholera phages were recovered. By contrast, a decline in the number of clinical cholera cases was associated with a marked increase in the prevalence of phages that were specific for the epidemic cholera strain.

Interestingly, during the study period, several environmental (non-pathogenic) V. cholerae strains were found to carry serotype-specific temperate phages, and the authors propose that the release of these phages by environmental strains might control cholera during the period between epidemics.

Finally, Mekalanos and his team propose a model in which cycles of phage amplification and predation might explain the seasonality of this disease.